The invention relates to a high-pressure gas discharge lamp (HID [high intensity discharge] lamp) which is in particular free from mercury and suitable for use in automobile technology.
Conventional high-pressure gas discharge lamps contain on the one hand a discharge gas (usually a metal halide such as sodium iodide or scandium iodide) which is the actual light-emitting material (light generator), and on the other hand mercury which primarily serves to form a voltage gradient and has the essential function of enhancing the efficacy and burning voltage of the lamp.
Lamps of this kind have come into widespread use because of their good properties and they are increasingly applied also in the field of automobile technology. It is also partly required in particular for this application, however, that the lamps should contain no mercury for environmental reasons.
EP 1,063,681 describes a discharge lamp in which the use of mercury in the discharge space is dispensed with so as to avoid environmental pollution and to reduce the proportion of ultraviolet radiation. It is further described therein, however, that such lamps cannot be used in motor vehicles because they do not generate the required luminous flux quickly enough after switching on. This problem is to be solved in that the heat capacity and the heat losses of the discharge space are reduced, and in that certain ratios between the internal gas pressure at room temperature on the one hand and the volume of the discharge space and its maximum wall thickness in several respective cross-sectional planes in the discharge space are to be observed.
It is a disadvantage here, however, that a reduced distance between the vessel inner wall and the light arc curved upwards by convection causes the light-generating substances to react with the silicon oxide, in particular in the comparatively hot upper wall region of the discharge vessel, thus leading to an intensified crystallization which shortens lamp life. In addition, such a lamp has a lower luminous efficacy than a lamp containing mercury.
Besides the lower luminous efficacy, there is a further, general problem in lamps without mercury, i.e. that a lower burning voltage and accordingly a higher lamp current are obtained for the same lamp power in continuous operation.
Finally, the application in the field of automobiles means that the constructional situation and the interaction with the reflector are to be taken into account, which means that the external shape of the lamp should remain substantially unchanged.
It is accordingly an object of the invention to provide a high-pressure gas discharge lamp which, with a mercury-free gas filling, is capable of achieving a luminous efficacy which corresponds substantially to that of lamps containing mercury, or whose efficacy in the case of a gas filling with mercury can be further enhanced.
A further object is to provide a high-pressure gas discharge lamp which, also with a gas filling without mercury, has a higher burning voltage than can be normally achieved with lamps free from mercury.
In particular, a high-pressure gas discharge lamp is to be provided by means of which at least one of the two objects mentioned above (higher luminous efficacy and higher burning voltage) can be achieved without the necessity of increasing the lamp power or the maximum thermal load thereon, or enlarging the outer dimensions of the outer bulb of the lamp.
A mercury-free high-pressure gas discharge is also to be provided which has a lumen maintenance usual for motor vehicle applications, i.e. which has a luminous decrement through lamp life which is similar to that of lamps with mercury.
Finally, a high-pressure gas discharge lamp is to be provided which is suitable for use in particular in automobile technology.
The object is achieved, according to claim 1, by means of a high-pressure gas discharge lamp with a discharge vessel which encloses a discharge space with a light-generating substance and with a bottom surface which is in lowermost position in the operational position of the lamp, which bottom surface has a first raised region and at least one second region, wherein the distance of the first region to an arc discharge formed during operation of the lamp is dimensioned such that the light-generating substance collected on said first region enters the gaseous state in a sufficient quantity owing to heating after switching on of the lamp, and wherein the second region is arranged such that it acts as a collector reservoir for the light-generating substance moving about owing to the heating caused by switching-on of the lamp. Such a reservoir is advantageous for several reasons. First, it prevents at least partly that migrating light-generating substances enter the regions of entry locations of the electrodes. Second, such a reservoir ensures that the molten salts disappear substantially from the first region, where they would hamper the emission of light from the lamp owing to reflection and absorption.
The second region may be arranged such here that it is lowered or raised with respect to the first region, or substantially forms a continuation of the first regionxe2x80x94in as far as it has a correspondingly large surface area. This arrangement is to be decided on primarily in dependence on the nature and the quantity of the light-generating substance introduced into the discharge vessel, i.e. the evaporation and mobility properties thereof.
A particular advantage of this solution is that said geometry of the discharge space renders it possible to raise the temperature of the coldest spots in the discharge space without the consequence that the maximum temperature (at the upper side in the operational position) and the maximum thermal load on the lamp rise, or that light-generating substances can move into the pinches and cause damage there.
This has among its results that either mercury can be omitted without substitute, or that a different voltage gradient forming material can be used instead of mercury, which material is less damaging to the environment, for example a suitable metal halide, while in all cases the light-generating substances enter the gas phase in sufficient quantities because of the higher temperature of the coldest spots achieved with this geometry, whereby the luminous efficacy of the lamp and its burning voltage are further enhanced. This may also be achieved through the introduction of a rare gas (in particular xenon), by which the gas pressure in the discharge space is raised.
It should be noted here that U.S. Pat. No. 5,211,595 discloses a method of manufacturing a discharge lamp with asymmetrical pinches and with an asymmetrical discharge vessel. The object of this asymmetry, in contrast to the solution according to the present invention, is to achieve a greater distance between the upper region of the discharge vessel and the light arc which is comparatively strongly upwardly curved during operation, so that said region is not heated so strongly. A further object is, in the manufacture of a discharge vessel with such an asymmetry and only a slight curvature in the lower wall region, to avoid cracks in said lower wall region, in which cracks metal halides would accumulate more strongly, and a larger surface is to be provided for facilitating the evaporation of said substances and for improving the photometric properties of the lamp. Finally, this lamp is not free from mercury, nor is it designed for use in automobile technology, so that this publication is not to be regarded as relevant.
The dependent claims relate to advantageous further embodiments of the invention.
The embodiments defined in claims 2 and 3 have the advantage in particular that the manufacture is particularly simple and accordingly inexpensive.
The embodiment defined in claim 4 is particularly suitable for use in motor vehicle headlights.
Claims 5 and 6 relate to voltage gradient forming materials which are to be used in preference to mercury and by means of which a particularly good luminous efficacy of the lamp can be achieved, while claim 7 provides an alternative possibility for achieving this object, and in particular for achieving a higher luminous efficacy and burning voltage.
The embodiment defined in claim 8 achieves a particularly homogeneous temperature rise of the bottom surface, while according to claim 9 the protection of the entry locations of the electrodes and the pinches situated behind them against the light-generating substances can be further improved.